1. Field of the Invention
The present invention relates to an inverter for supplying a fluorescent lamp and the like with drive voltage.
2. Description of the Related Art
In recent years, flat-screen LCD televisions growing in size and flatter in thickness are in wide use replacing the CRT televisions. In the LCD television, a plurality of cold cathode fluorescent lamps (hereinafter referred to as CCFL) or external electrode fluorescent lamps (hereinafter referred to as EEFL) are placed on the back surface of LCD panels on which images are displayed, and they emit light as backlight.
An inverter (DC/AC converter), which boosts DC voltage of, for example, about 12 V and outputs it as AC voltage, is used to drive CCFL or EEFL. The inverter converts the current flowing through CCFL, to a voltage and then feeds it back to a control circuit. Based on this fed-back voltage, the inverter controls the on and off of switching elements. For example, Patent Document 1 discloses a technology for driving fluorescent lamps by such an inverter.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-323994.
1. In a case when a load such as a fluorescent lamp is driven at a high voltage, important is a technique for protecting a circuit against the circuit failures such as overcurrent, overvoltage, short-to-ground (ground short) and short-to-supply (source short). As for such circuitry protection, a method is generally implemented where the switching operation of an inverter is stopped when a circuit failure continues for a predetermined time period.
Nevertheless, there are cases where in this method the inverter continues to operate until a predetermined time period has elapsed after the circuit failure occurred and therefore such a technique is insufficient to protect the circuitry. In particular, if the fluorescent lamps are connected in parallel and driven, the circuit current will be very large and power consumed will rise to about several hundreds of W. Thus, protecting human bodies against earth fault and the like must be ensured.
To perform dimming control of adjusting the luminance of a fluorescent lamp, a control circuit in the inverter may be provided with a dimming function. There are two types of the dimming. One is a case where the dimming is set by a set maker that designs equipment on which the fluorescent lamps and inverters are mounted. The other is a case where the dimming is set when a user uses the equipment. As these dimming means, there are an analog dimming control (current dimming control), a burst dimming control where the fluorescent lamp emits light intermittently, and so forth.
The control circuit in the inverter circuit performs the above-described dimming and simultaneously protects the circuits by detecting the circuit failure such as non-lighting, overvoltage and overcurrent. Here, when the light controlling level set by the set maker or the user is to be maintained in the event that a circuit fault occurs in the inverter, problems may arise in the circuit operation. For example, when the inverter is short-circuited to the ground, driving the inverter to maintain the light control level causes a problem where the current continues to flow through a ground-short path.
A lamp such as EEFL generally emits light by utilizing the resonance of an leakage inductance and a parasitic capacitance of the lamp itself. Its resonance frequency fr is given by fr=1/(2×π×√{square root over (LC)}) using an inductance L and a capacitance C.
Here, the parasitic capacitance of the lamp before a start of discharging is higher than that after the start of discharging. Thus, the resonance frequency changes at the time when the lamp emits light and when it does not. Consequently, it is desired that the control circuit in the inverter shall vary a switching frequency of a voltage applied to a transformer according to a lighting or non-lighting state of the lamp.    1. One embodiment of the present invention has been made in view of the problems described as above, and an advantage thereof is to provide a control circuit of an inverter that strengthens a protective function against circuit failures.    2. An advantage of another embodiment of the present invention is to provide a control circuit of an inverter capable of restricting a light control from outside.    3. An advantage of still another embodiment of the present invention is to provide a control circuit of an inverter capable of controlling a switching frequency in accordance with a state of a fluorescent lamp to be driven.
1. One embodiment of the present invention relates to a control circuit for controlling an inverter including a transformer. This control circuit comprises: a pulse modulator which generates a pulse signal where a duty ratio thereof is feedback-controlled in a manner such that a detection voltage corresponding to a current across a secondary coil of the transformer is brought close to a reference voltage; a logic control unit which performs a switching control of the current across the primary coil of the transformer, based on the pulse signal outputted from the pulse modulator; a first protection circuit which detects a circuit failure of the inverter and which stops the switching control of the inverter when the circuit failure continues for a predetermined duration of error detection time; and a second protection circuit which monitors a feedback voltage corresponding to an output voltage of the inverter and which sets the duration of error detection time shorter and lowers the reference voltage when the feedback voltage is lower than a predetermined threshold voltage.
By implementing this embodiment, when error is detected by the first protection circuit and the output voltage of the inverter drops due to short-circuiting of a load or the like, a period until when the switching control will be stopped is shortened. Also, the output current of the inverter is forcibly reduced and the power consumption is reduced by lowing the reference voltage, so that the circuitry protection can be strengthened.
The second protection circuit may include: a voltage source which generates the threshold voltage; and a second comparator which compares the feedback voltage with the threshold voltage generated by the voltage source. During a start-up period after the control circuit is started, the voltage source may set the threshold voltage lower than that at normal operating time after the start-up period has elapsed.
In the case of performing a soft-start that raises slowly the output voltage after a start-up, the threshold voltage of the second protection circuit is set low during a start-up period after a start when the output voltage is low. A distinction can be made between a state where the output voltage is low due to the short-circuiting and a state where the output voltage is low due to the soft-start. Hence, the start-up period can be excluded from the circuit protection executed by the second protection circuit.
The first protection circuit may include: a capacitor whose potential at one end thereof is fixed; a current source which generates a charging current and charges the capacitor; and a first comparator which compares a voltage appearing across the capacitor with a voltage corresponding to the duration of error detection time. When the feedback voltage is lower than the threshold voltage, the second protection circuit may increase the charging current.
The pulse modulator, the logic control unit and the first and the second protection circuit may be integrated on a single semiconductor substrate. “Being integrated” includes a case where all of circuit components are formed on a semiconductor substrate and a case where the main components of a circuit are integrated thereon. Note that part of resistors or capacitors used to adjust circuit constants may be provided outside the semiconductor substrate. Integrating the control circuit as a single LSI can reduce the circuit area.
Another embodiment of the present invention relates to an inverter. This inverter comprises: a transformer; an above-described control circuit which performs a switching control on current across a primary coil of the transformer; and a current-voltage conversion unit, provided on a current path of a secondary coil of the transformer, which converts current flowing through the secondary coil into voltage so as to be fed back to the control circuit as the detection voltage.
Still another embodiment of the present invention relates to a light emitting device. This light emitting device comprises: a fluorescent lamp; and an above-described inverter which supplies output voltage to the fluorescent lamp as a drive voltage. Also, a plurality of the fluorescent lamps may be connected in parallel. Also, two of the inverters may be provided at both ends of the fluorescent lamp, respectively, and may supply drive voltages of mutually reversed phases to the fluorescent lamp. Also, the fluorescent lamp may be a cold cathode fluorescent lamp or may be an external electrode fluorescent lamp.
Still another embodiment of the present invention relates to a liquid-crystal television. This liquid-crystal television comprises: a liquid-crystal panel; and a plurality of light emitting devices arranged on a backside of the liquid-crystal panel.
2. Another embodiment of the present invention relates to a control circuit of an inverter for driving a fluorescent lamp. This control circuit comprises: a voltage source which generates a predetermined reference voltage; an error amplifier which amplifies error between either an analog dimming control voltage inputted externally to adjust the luminance of a fluorescent lamp to be driven or the reference voltage generated by the voltage source, whichever is lower, and a detection voltage according to a current flowing through a secondary coil of a transformer in the inverter; a pulse-width-modulation comparator which compares error voltage outputted from the error amplifier with a triangular wave voltage so as to output a pulse-width-modulation signal; and a logic control unit which performs a switching control of the current across the primary coil of the transformer, based on the pulse-width-modulation signal outputted from the pulse-width-modulation comparator.
According to this embodiment, when the analog dimming control voltage is lower than the reference voltage, a feedback is provided so that the detection voltage is brought close to the analog dimming control voltage. This can realize a light control from the outside. Also, when the analog dimming control voltage becomes higher than the reference voltage, a feedback is provided so that the detection voltage is brought close to the reference voltage. This can cause the fluorescent lamp to emit light at luminance predetermined by the control circuit. As a result, the current flowing through the secondary coil of the transformer can be restricted to a current value determined by the reference voltage or below it.
A control circuit may further comprise a protection circuit which monitors a feedback voltage according to an output voltage of the inverter and which lowers the reference voltage when the feedback voltage is lower than a predetermined threshold voltage.
In such a case, if the output voltage drops due to error such as ground-short, the reference voltage will be lowered. Thus, an upper limit of current flowing through the secondary coil of the transformer can be lowered, thus strengthening the circuitry protection.
A control circuit may further comprise an analog dimming stop switch provided between an input terminal of the analog dimming control voltage and the error amplifier. The analog dimming stop switch may be turned off when the fluorescent lamp to be driven does not emit light.
When the analog dimming is performed while the fluorescent lamp is not lit, it is further unlikely for the fluorescent to emit light. Consequently, the analog dimming stop switch is turned off when the fluorescent lamp is not lit. This invalidates the analog dimming and therefore a feedback is provided so that the current determined by the reference voltage flows through the fluorescent lamp, thus facilitating the emission of light.
When the feedback voltage is lower than the threshold voltage, the protection circuit may turn off the analog dimming stop switch.
In this case, if error such as ground-short is detected by the protection circuit, the analog dimming control voltage will not be inputted to the error amplifier. Hence, a feedback is provided so that the current determined by the reference voltage flows to a lamp. This can facilitate the lamp to emit light.
A control circuit may further comprise: a burst dimming comparator which compares a burst dimming control voltage inputted externally with a second triangular wave voltage whose frequency is set lower than that of the triangular wave voltage; and a forced off circuit which refers to an output signal of the burst dimming comparator and raises forcibly the detection voltage to a voltage value, where a duty ratio of the pulse-width-modulation signal becomes practically 0, during a period of which the burst dimming control voltage is lower than the second triangular wave voltage.
A “voltage value where a duty ratio of the pulse-width-modulation signal becomes practically zero” means a range where the switching of the current flowing through the primary coil of the transformer stops. In this case, the drive voltage is supplied intermittently to the fluorescent lamp according to the level of the output signal of the burst dimming comparator, so that the luminance of the fluorescent lamp can be adjusted.
When the feedback voltage is lower than the threshold voltage, the protection circuit may set the forced off circuit inactive and stop burst dimming.
In this case, if error such as ground-short is detected by the protection circuit, the light emission of the fluorescent lamp can be facilitated by stopping the burst light control.
The voltage source, the error amplifier, the pulse-width-modulation comparator and the logic control unit may be integrated on a single semiconductor substrate. Integrating the control circuit as a single LSI can reduce the circuit area.
Another embodiment of the present invention relates to an inverter. This inverter comprises: a transformer in which a fluorescent lamp to be driven is connected to a secondary coil thereof; a control circuit which performs a switching control on current across a primary coil of the transformer; and a current-voltage conversion unit, provided on a current path of the secondary coil of the transformer, which converts current flowing through the secondary coil into voltage so as to be fed back to the control circuit as the detection voltage.
Still another embodiment of the present invention relates to a light emitting device. This light emitting device comprises: a fluorescent lamp; and an inverter which supplies output voltage to the fluorescent lamp as a drive voltage. A plurality of the fluorescent lamps may be connected in parallel. Also, two of the inverters may be provided at both ends of the fluorescent lamp, respectively, and may supply drive voltages of mutually reversed phases to the fluorescent lamp. Further, the fluorescent lamp may be an external electrode fluorescent lamp or may be a cold cathode fluorescent lamp.
Still another embodiment of the present invention relates to a liquid-crystal television. This liquid-crystal television comprises: a liquid-crystal panel; a plurality of light emitting devices arranged on a backside of the liquid-crystal panel; and a signal processing unit which outputs the analog dimming control voltage to the inverter in the light emitting device.
3. Another embodiment of the present invention relates to a control circuit of an inverter for driving a fluorescent lamp. This control circuit comprises: an error amplifier which amplifies error between a dimming control voltage to adjust the luminance of a fluorescent lamp to be driven and a detection voltage according to a drive current flowing through a secondary coil of a transformer in the inverter; a triangular wave signal generator which generates a triangular wave signal; a pulse-width modulation-comparator which compares error voltage outputted from the error amplifier with the triangular wave voltage outputted from the triangular wave signal generator so as to output a pulse-width-modulation signal; a logic control unit which performs a switching control of the current across the primary coil of the transformer, based on the pulse-width-modulation signal outputted from the pulse-width-modulation comparator; and a frequency control unit which monitors detection voltages according to the drive current flowing through the secondary coil of the transformer and a drive voltage supplied to the fluorescent lamp from the inverter, respectively, and which raises the frequency of the triangular wave signal when a first detection voltage according to the drive current is lower than a first predetermined threshold voltage or a second detection voltage according to the drive voltage is higher than a second predetermined threshold voltage.
According to this embodiment, a state where the drive current is lower than a predetermined value or a state where the drive voltage is higher than a predetermined level is determined to be a state where the fluorescent lamp is not lit, and the frequency of the triangular wave signal is raised. Thus, the light-emitting property can be enhanced.
The frequency control unit may include: a first comparator which compares the first detection voltage according to the drive voltage with the first threshold voltage and which sets an output to a predetermined level when the first detection voltage is lower than the first threshold voltage; a second comparator which compares the second detection voltage according to the drive current with the second threshold voltage and which sets an output to a predetermined level when the second detection voltage is higher than the second threshold voltage; and a logic gate which performs a logical operation of the output signals of the first and the second comparator, wherein the frequency control unit may control the frequency of the triangular wave signal by an output signal of the logic gate.
The first comparator may set either a voltage proportional to the dimming control voltage or a predetermined reference voltage, whichever is lower, to the first threshold voltage with which the first detection voltage is compared.
In this case, when the voltage proportional to the dimming control voltage becomes lower than the reference voltage, the first threshold voltage to be compared with the first detection voltage varies according to the dimming control voltage. Thus, even when the fluorescent lamp is to emit light with low luminance, the lighting and non-lighting of the lamp can be suitably detected.
The triangular wave signal generator may include: a capacitor; and a charge-discharge circuit which supplies a charging current to the capacitor and pulls out a discharging current from the capacitor. The frequency control unit may raise the frequency of the triangular wave signal by increasing the charging current and the discharging current of the charge-discharge circuit.
The control circuit may be integrated on a single semiconductor substrate. Integrating the control circuit as a single LSI can reduce the circuit area.
Another embodiment of the present invention relates to an inverter equipped with the above-described control circuit. This inverter comprises: a transformer in which a fluorescent lamp to be driven is connected to a secondary coil thereof; a above-described control circuit which performs a switching control on current across a primary coil of the transformer; a drive voltage detection unit which converts the drive voltage, supplied to the fluorescent lamp to be driven, into a direct-current voltage by half-wave rectifying the drive voltage and feeds back the voltage to the control circuit as the first detection voltage; and a current-voltage conversion unit, provided on a current path of the secondary coil of the transformer, which converts drive current flowing through the secondary coil into voltage so as to be fed back to the control circuit as the second detection voltage.
According to this embodiment, the drive voltage and drive current for the fluorescent lamp are fed back to the control circuit so as to determine the light or non-lighting state of the fluorescent. Thus the drive frequency of the transformer can be optimally switched.
Still another embodiment of the present invention relates to a light emitting device. This light emitting device comprises: a fluorescent lamp; and an above-described inverter which supplies output voltage to the fluorescent lamp as a drive voltage.
According to this embodiment, the light-emitting property of the fluorescent lamp can be improved.
A plurality of the fluorescent lamps may be connected in parallel. Two of the inverters may be provided at both ends of the fluorescent lamp, respectively, and may supply drive voltages of mutually reversed phases to the fluorescent lamp. The fluorescent lamp may be an external electrode fluorescent lamp or may be a cold cathode fluorescent lamp.
Still another embodiment of the present invention relates to a liquid-crystal television. This liquid-crystal television comprises: a liquid-crystal panel; and a plurality of light emitting devices arranged on a backside of the liquid-crystal panel.
According to this embodiment, the light-emitting property of the fluorescent lamp used as backlight of liquid crystal can be enhanced.
It is to be noted that any arbitrary combination or rearrangement of the above-described structural components and so forth is effective as and encompassed by the present embodiments.
Moreover, this summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.